Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.

Thallus: squamulose squamules: large (up to 10 mm wide and up to 0.6 mm thick) and leafy, mostly densely aggregated and overlapping, roundish or with broadly rounded lobes, ± wavy with margins free from the substrate upper surface: medium to dark chestnut brown or red-brown, dull or glossy upper cortex: 40-80 µm; epinecral layer: varying from almost lacking to 40 µm thick medulla: white, 100-300 µm thick, composed of intricately interwoven filamentous hyphae, usually without (or with very few) spherical cells; algal layer: c. 100 µm thick lower cortex: distinct, paraplectenchymatous, composed of roundish-angular cells (13-20 µm in diam.) lower surface: pale to dark brown, blackening centrally; rhizohyphae: hyaline, c. 6-7.5 µm thick, lacking in a ± broad marginal zone Perithecia: broadly pyriform, up to 0.5 mm broad, fully immersed in the squamules but without bulging the lower sides; periphyses: 40-60 x 3.5-5 µm asci: cylindrical, 80-100 x 12-19 µm, 8-spored ascospores: uniseriate, ellipsoid, c. 15-20 x 7.5-9.5 µm Pycnidia: marginal, knob-like conidia: ellipsoid-oblong, 3-5 x 1.5-2 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on soil, plant debris and moss, also directly on (calcareous) rock World distribution: temperate Northern Hemisphere; Europe, Asia, North Africa, rare in North America Sonoran distribution: rare in upland Arizona. Notes: Normally Placidium rufescens is easily identifiable because of the following combination of characters: large, ± wavy squamules with a thick prosoplectenchymatous medullary tissue, thick rhizohyphae, large ascospores, and marginal pycnidia with ellipsoid-oblong conidia. However, forms with many spherical cells in the medulla do occur, but the large spores and pycnidial characters are distinctive. Placidium chilense is anatomically very similar but has smaller spores and laminal pycnidia.